So imagine if I threw glue at you, and that glue lasts for 30 seconds. Let's say some monster comes up to you and punches you in the face. There's a 20% chance that his fist will be covered in glue for 20 seconds. Let's say another monster comes up and punches you in the face. There's also a 20% chance that his fist will be covered in glue for 20 seconds as well.

So let's say a whole army of monsters come up to you and whacks you in the face. They all have a 20% chance of having their fists all glued up for 20 seconds. They're all independent from that 30 second duration. That 20% chance will keep occurring until your buff wears off, which then you will no longer apply the debuff if you get punched in the face but all existing debuffs on the other monsters will persist until their duration runs out.

MNK: Woosh! Woosh! Now I'm behind you! Now I'm on the side! OOP Maybe I can squeeze in this attack in before the AoE hits~! YEAH! LOOK AT THE DAMAGE! WOO-- oh wait the boss jumped... Well there goes that built up momentum. Gotta start all over D: Kick em on the side, punch em in the butt, always moving, always alternating, always be punching!

DRG: Poke poke! Gotta get my Heavy Thrust in for more damage! Have an "up-the-butt" combo on the house so I can do EVEN MORE DAMAGE. Jumps?! I got them SON! BLAM! BLAM! FIREY BLAM... oh uh... dammit I got animation locked and couldn't dodge the AoE because of the jumps. :( Raise pls. YEAH I'M BACK UP! Good thing I don't have to rebuild much to get back into the brawl~

BRD: PEW PEW PEW! I CAN RUN AROUND ALL DAY AND DPS AND YOU CAN'T DO ANYTHING ABOUT IT! LOOK AT ME, LEMME MASH MY FACE AGAINST THE KEYBOARD AND STILL DO HIGH DPS! WOOoooo wait you need Ballad? >:( How'd you lose all that MP?

WHM: pulls out a hammer and plays whack a mole with HP bars D: Well... this is boring. pulls out Cleric Stance and starts DPSing I SHALL BE THE ONE TO FILL YOUR DARK SOUL WITH LIIIIIGGGGHHHHHTTTT ... oh uhm... mp song please.

SCH: Man I have it all~ I got my fairy doing the healing work, I got unlimited MP, and as long as I pay attention, everyone's health will be kept up because I got them shields and emergency lustrates to keep the tank and everyone alive... goes into Cleric Stance and DoTs on DoTs on DoTs Oh no I'm low of mana an- smashes energy drain and Aetherflow to restore mana LET'S GO.

In a nutshell.

As far as other questions go: BRD and SMN are two classes that don't really follow a super strict rotation, rather a priority system of "this DoT/buff is wearing off, re-apply it". However, MNK and DRG don't have strict rotations in that there will be times where you have to think ahead about reapplying DoT/buffs. There's nothing really too static, even BLM with its random procs that changes up the variety of it all.

As far as combos go: Combos are your recommended rotations as one ability will link with another ability to do more damage. Not like FFXI skillchains at all in that you don't interact with other people in that regard and especially not in a "optional" bonus, as they are mandatory to keep your DPS on par.

I hope my description of the feels aren't TOO abstract. D:

Edit: Whoops, added the end of the WHM part to SMN by accident. We know you folks don't have mana issues... pfft!

FFXI was hard because you had to buy padded stuff for things in your room for that time where someone calls you up at 3 in the morning to go tank Tiamat for 5 hours with only 4 other people, in which you promptly grab your cell phone and chuck it against the wall.